Indigo dye is an organic compound with a distinctive blue color (see indigo). Historically, indigo was a natural dye extracted from plants, and this process was important economically because blue dyes were once rare. Nearly all indigo dye produced today – several thousand tons each year – is synthetic. It is the blue of blue jeans.

Uses[edit]

Indigo dye

The primary use for indigo is as a dye for cotton yarn, which is mainly for the production of denimcloth for blue jeans. On average, a pair of blue jean trousers requires 3–12 g of indigo. Small amounts are used for dyeing wool and silk.

Indigo carmine, or indigotine, is an indigo derivative which is also used as a colorant. Approximately 20M kilograms are produced annually, again mainly for blue jeans.[1] It is also used as a food colorant, and is listed in the United States as FD&C Blue No. 2, and in the European Union as E NumberE132.

Natural indigo[edit]

Plant sources[edit]

A variety of plants have provided indigo throughout history, but most natural indigo was obtained from those in the genus Indigofera, which are native to the tropics. The primary commercial indigo species in Asia was true indigo (Indigofera tinctoria, also known as Indigofera sumatrana). A common alternative used in the relatively colder subtropical locations such as Japan's Ryukyu Islands and Taiwan is Strobilanthes cusia (Japanese: リュウキュウアイ. Chinese: 馬藍/山藍). In Central and South America the two species Indigofera suffruticosa (Añil) and Dyer's Knotweed (Polygonum tinctorum), although the Indigofera species yield more dye.

Extraction[edit]

The precursor to indigo is indican, a colorless, water-soluble derivative of the amino acid tryptophan. Indican readily hydrolyzes to release β-D-glucose and indoxyl. Oxidation by exposure to air converts indoxyl to indigo. Indican was obtained from the processing of the plant's leaves, which contain as much as 0.2–0.8% of this compound. The leaves were soaked in water and fermented in order to convert the glycoside indican present in the plant to the blue dye indigotin.[2] The precipitate from the fermented leaf solution was mixed with a strong base such as lye, pressed into cakes, dried, and powdered. The powder was then mixed with various other substances to produce different shades of blue and purple.

Cultivation[edit]

Indigo was a major export crop that supported plantation slavery in colonial South Carolina in the 18th century.[3]

The demand for indigo in the 19th century is indicated by the fact that in 1897, 7000 square kilometers were dedicated to the cultivation of indican-producing plants, mainly in India. By comparison, the country of Luxembourg is 2,586 square kilometers.[1]

History of natural indigo[edit]

Indigo was used in India, which was also the earliest major center for its production and processing.[4] The Indigofera tinctoria variety of Indigo was domesticated in India.[4] Indigo, used as a dye, made its way to the Greeks and the Romans, where it was valued as a luxury product.[4]

India is believed to be the oldest center of indigo dyeing in the Old World. It was a primary supplier of indigo to Europe as early as the Greco-Roman era. The association of India with indigo is reflected in the Greek word for the dye, indikón (ινδικόν, Indian). The Romans latinized the term to indicum, which passed into Italian dialect and eventually into English as the word indigo.

Cake of indigo. Large piece approximately 2 cm. Used as indigo dye.

In Mesopotamia, a Neo-Babylonian cuneiform tablet of the 7th century BC gives a recipe for the dyeing of wool, where lapis-colored wool (uqnatu) is produced by repeated immersion and airing of the cloth. Indigo was most probably imported from India. The Romans used indigo as a pigment for painting and for medicinal and cosmetic purposes. It was a luxury item imported to the Mediterranean from India by Arab merchants.

Indigo remained a rare commodity in Europe throughout the Middle Ages. Woad, a chemically identical dye derived from the plant Isatis tinctoria (Brassicaceae), was used instead. In the late 15th century, the Portuguese explorer Vasco da Gama discovered a sea route to India. This led to the establishment of direct trade with India, the Spice Islands, China, and Japan. Importers could now avoid the heavy duties imposed by Persian, Levantine, and Greek middlemen and the lengthy and dangerous land routes which had previously been used. Consequently, the importation and use of indigo in Europe rose significantly. Much European indigo from Asia arrived through ports in Portugal, the Netherlands, and England. Spain imported the dye from its colonies in South America. Many indigo plantations were established by European powers in tropical climates; it was a major crop in Jamaica and South Carolina, with much or all of the labor performed by enslaved Africans and African-Americans. Indigo plantations also thrived in the Virgin Islands. However, France and Germany outlawed imported indigo in the 16th century to protect the local woad dye industry.

Indigo was the foundation of centuries-old textile traditions throughout West Africa. From the Tuareg nomads of the Sahara to Cameroon, clothes dyed with indigo signified wealth. Women dyed the cloth in most areas, with the Yoruba of Nigeria and the Mandinka of Mali particularly well known for their expertise. Among the Hausa male dyers, working at communal dye pits was the basis of the wealth of the ancient city of Kano, and they can still be seen plying their trade today at the same pits.[5]

In Japan, indigo became especially important in the Edo period when it was forbidden to use silk, so the Japanese began to import and plant cotton. It was difficult to dye the cotton fiber except with indigo. Even today indigo is very much appreciated as a color for the summer Kimono Yukata, as this traditional clothing recalls Nature and the blue sea.

In North America indigo was introduced into colonial South Carolina by Eliza Lucas Pinckney, where it became the colony's second most important cash crop (after rice).[6] When Benjamin Franklin sailed to France in November 1776 to enlist France's support for the American Revolutionary War there were 35 barrels of indigo on board the Reprisal, the sale of which would help fund the war effort.[7] In colonial North America there were three commercially important species: the native Indigofera caroliniana, and the introduced Indigofera tinctoria and Indigofera suffruticosa.[8]

Newton used "indigo" to describe one of the two new primary colors he added to the five he had originally named, in his revised account of the rainbow in Lectiones Opticae of 1675.[9]

Because of its high value as a trading commodity, indigo was often referred to as Blue Gold.

Era of synthetic indigo[edit]

Production of Indigo dye in a BASF plant (1890)

In 1897, 19,000 tons of indigo were produced from plant sources. Largely due to advances in organic chemistry, production by natural sources dropped to 1,000 tons by 1914 and continued to contract. These advances can be traced to 1865 when the German chemist Adolf von Baeyer began working on the synthesis of indigo. He described his first synthesis of indigo in 1878 (from isatin) and a second synthesis in 1880 (from 2-nitrobenzaldehyde). The synthesis of indigo remained impractical, so the search for alternative starting materials at BASF and Hoechst continued. The synthesis of N-(2-carboxyphenyl)glycine from the easy to obtain aniline provided a new and economically attractive route. BASF developed a commercially feasible manufacturing process that was in use by 1897. In 2002, 17,000 tons of synthetic indigo were produced worldwide.

Developments in dyeing technology[edit]

Indigo white (leuco-indigo)

Yarn dyed with indigo dye

Indigo white[edit]

Indigo is a challenging dye because it is not soluble in water. To be dissolved, it must undergo a chemical change (reduction). Reduction converts indigo into "white indigo" (leuco-indigo). When a submerged fabric is removed from the dyebath, the white indigo quickly combines with oxygen in the air and reverts to the insoluble, intensely colored indigo. When it first became widely available in Europe in the 16th century, European dyers and printers struggled with indigo because of this distinctive property. It also required several chemical manipulations, some involving toxic materials, and had many opportunities to injure workers. In the 19th century, English poet William Wordsworth referred to the plight of indigo dye workers of his hometown of Cockermouth in his autobiographical poem "The Prelude". Speaking of their dire working conditions and the empathy that he feels for them, he wrote,

Doubtless, I should have then made common cause

With some who perished; haply perished too

A poor mistaken and bewildered offering

Unknown to those bare souls of miller blue

A preindustrial process for production of indigo white, used in Europe, was to dissolve the indigo in stale urine. A more convenient reductive agent is zinc. Another preindustrial method, used in Japan, was to dissolve the indigo in a heated vat in which a culture of thermophilic, anaerobic bacteria was maintained. Some species of such bacteria generate hydrogen as a metabolic product, which convert insoluble indigo into soluble indigo white. Cloth dyed in such a vat was decorated with the techniques of shibori (tie-dye), kasuri, katazome, and tsutsugaki. Examples of clothing and banners dyed with these techniques can be seen in the works of Hokusai and other artists.

Direct printing[edit]

Two different methods for the direct application of indigo were developed in England in the 18th century and remained in use well into the 19th century. The first method, known as pencil blue because it was most often applied by pencil or brush, could be used to achieve dark hues. Arsenic trisulfide and a thickener were added to the indigo vat. The arsenic compound delayed the oxidation of the indigo long enough to paint the dye onto fabrics.

Pot of freeze-dried indigo dye

The second method was known as china blue due to its resemblance to Chinese blue-and-white porcelain. Instead of using an indigo solution directly, the process involved printing the insoluble form of indigo onto the fabric. The indigo was then reduced in a sequence of baths of iron(II) sulfate, with air-oxidation between each immersion. The china blue process could make sharp designs, but it could not produce the dark hues possible with the pencil blue method.

Around 1880 the glucose process was developed. It finally enabled the direct printing of indigo onto fabric and could produce inexpensive dark indigo prints unattainable with the china blue method.

Since 2004 freeze-dried indigo, or instant indigo, has become available. In this method the indigo has already been reduced, and then freeze-dried into a crystal. The crystals are added to warm water to create the dye pot. As in a standard indigo dye pot, care has to be taken to avoid mixing in oxygen. Freeze-dried indigo is simple to use, and the crystals can be stored indefinitely as long as they are not exposed to moisture.[10]

The molecule absorbs light in the orange part of the spectrum (λmax = 613 nm).[11] The compound owes its deep color to the conjugation of the double bonds, i.e. the double bonds within the molecule are adjacent and the molecule is planar. In indigo white, the conjugation is interrupted because the molecule is nonplanar.

Chemical synthesis[edit]

Given its economic importance, indigo has been prepared by many methods. The Baeyer-Drewson indigo synthesis dates back to 1882. It involves an aldol condensation of o-nitrobenzaldehyde with acetone, followed by cyclization and oxidative dimerization to indigo. This route is highly useful for obtaining indigo and many of its derivatives on the lab-scale, but was impractical for industrial-scale synthesis. The first commercially-practical route is credited to Pfleger in 1901. In this process, N-phenylglycine is treated with a molten mixture of sodium hydroxide, potassium hydroxide, and sodamide. This highly sensitive melt produces indoxyl, which is subsequently oxidized in air to form indigo. Variations of this method are still in use today. An alternative and also viable route to indigo is credited to Heumann in 1897. It involves heating N-(2-carboxyphenyl)glycine to 200 °C in an inert atmosphere with sodium hydroxide. The process is easier than the Pfleger method but the precursors are more expensive. Indoxyl-2-carboxylic acid is generated. This material readily decarboxylates to give indoxyl, which oxidizes in air to form indigo.[1] The preparation of indigo dye is practiced in college laboratory classes according to the original Baeyer-Drewsen route.[12]

Heumann's original synthesis of indigo

Pfleger's synthesis of indigo.

Indigo derivatives[edit]

Structure of Tyrian purple.

Structure of indigo carmine.

The benzene rings in indigo can be modified to give a variety of related dyestuffs. Thioindigo, where the two NH groups are replaced by S atoms, is deep red. Tyrian purple is a dull purple dye that is secreted by a common Mediterranean snail. It was highly prized in antiquity. In 1909 its structure was shown to be 6,6'-dibromoindigo. It has never been produced on a commercial basis. The related Ciba blue (5,7,5′,7′-tetrabromoindigo) is, however, of commercial value. Indigo and its derivatives featuring intra- and intermolecular hydrogen bonding have very low solubility in organic solvents. They can be made soluble by using transient protecting groups such as the tBOC group, which suppresses intermolecular bonding.[13] Heating of the tBOC indigo results in efficient thermal deprotection and regeneration of the parent H-bonded pigment.

Treatment with sulfuric acid converts indigo into a blue-green derivative called indigo carmine (sulfonated indigo). It became available in the mid-18th century. It is used as a colorant for food, pharmaceuticals, and cosmetics.

Indigo as an organic semiconductor[edit]

Indigo and some of its derivatives are known to be ambipolar organic semiconductors when deposited as thin films by vacuum evaporation.[14]

Safety and the environment[edit]

Indigo has a low oral toxicity, with an LD50 of 5000 mg/kg in mammals.[1] In 2009, large spills of blue dyes had been reported downstream of a blue jeans manufacturer in Lesotho.[15]